1. Field of the Invention
The present invention relates to a fixing device and a fixing method in which an image is fixed onto a recording material while supplying power to a heating part of a fixing roller from a power storage unit, and to an image forming apparatus including the fixing device.
2. Discussion of the Related Art
A fixing device that fixes a toner image formed on a recording material, such as a recording sheet, via a heating part having a main heating element and an auxiliary heating element has been widely used. In such a background fixing device, the main heating element is powered by a main power supply unit, and the auxiliary heating element is powered by an auxiliary power supply unit including a capacitor. When the heating part starts to generate heat, the amount of power supplied from the capacitor to the auxiliary heating element is adjusted based on the temperature of the heating part.
Published Japanese patent application No. 2002-184554 describes the above fixing device. In the fixing device, the heating part is rapidly warmed-up to a predetermined temperature by being supplied with a large amount of power from both the main power supply unit and the auxiliary power supply unit. During a stand-by state of the fixing device, the main power supply unit and the auxiliary power supply unit do not supply power to the heating part. Thus, a power-saving effect is enhanced, and noise caused by a sudden current change or an in-rush current at the time of starting or stopping the supply of high power is reduced. Further, a warm-up time of the heating part is reduced, and the heating part is prevented from overheating.
The fixing device further includes a charger, a switching unit, a temperature detecting unit, and a control unit. The charger charges the capacitor of the auxiliary power supply unit with power supplied from the main power supply unit. The switching unit performs switching between the charge of the auxiliary power supply unit and the supply of power from the auxiliary power supply unit to the auxiliary heating element. The temperature detecting unit detects the temperature of the heating part. The control unit controls the amount of power supplied from the auxiliary power supply unit to the auxiliary heating element based on the temperature of the heating part detected by the temperature detecting unit.
Published Japanese patent application No. 2002-268421 describes another fixing device including a center portion heater and an end portion heater. The center portion heater heats around a center portion of a fixing roller in its axial direction, and the end portion heater heats around end portions of the fixing roller in its axial direction. This fixing device makes a temperature distribution uniform in the longitudinal direction of the fixing roller to prevent occurrences of fixing unevenness, wrinkles on a recording material, and a hot offset condition in which a part of a fused toner image carried on a recording material adheres to the fixing roller.
When a plurality of recording materials each carrying an image consecutively pass through a fixing device, the recording materials absorb heat from a fixing roller, thereby decreasing the temperature of the fixing roller. Therefore, it is desirable to prevent the drop in temperature of the fixing roller in such a condition. Further, when a plurality of recording materials each carrying an image and having a small width in a direction perpendicular to a conveying direction of the recording material consecutively pass through the fixing device, the temperature of a center portion of the fixing roller in its axial direction decreases; however, end portions of the fixing roller in its axial direction where the recording material having the small width does not pass through typically overheat. Therefore, it is desirable to control the temperature of the end portions of the fixing roller to prevent overheating of the end portions when a plurality of recording materials each having a small width in a direction perpendicular to a conveying direction of the recording material consecutively pass through the fixing device.